Reducing zinciferous materials



NOV. lgl. F, G, BREYER 1,832,356

REDUC ING Z INCIFEROUS MATERIALS Original Filed Feb. 17. 1927 3 Sheets-Sheet l ATTORNEYS Nnv. 17, 1931. i F. G. BREYER 1,832,356

REDUCING ZINCIFEROUS MATERIALS Original Filed Feb. 17, 1927 3 Sheets-Sheet 5 INVENTOR I BY w MMg/,wmww/M ATTORNEYS toward the open or outer` end.

retort.

Patented Nov. 17, 1931 UNITED STATES PATENT oFFicE ,FRANK G. BREY'ER, OE NEW CASTLE, DELAWARE, ASSIGNOR TO THE NEW JERSEY ZINC Y COMPANY, 0F NEW YORK, N, Y., A CORPORATION OF NEW JERSEY REDUCING ZINCIFEROUS MATERIALS Original application med February 17, 19/27, Serial No. 168,914. Divided and this 22, 1929. Serial No. 349,081.

This invention relates to the reduction or smeltng of zincierous materials and has for its object the provision of certain im.-

provements in the reduction or smelting of such materials. t Zinc metal or spelter, when produced by the reduction ot' oxidized zinc ores at high temperatures, is almost universally made at the present time in zinc' distillation or spelter 'furnaces having a number of relatively small retorts to the outer ends of which small condensei's are attached. rllhe retorts are usually mounted at a slight inclination, usually ini clined downward from the butt or closed eiid rlhe retorts are made of heat refractory material, such as tire clay, silicon carbide or the like, and are externally heated by iire gases of a temperatui-e around 14000 to 15000 C. ln the opera- 23 tion of these furnaces, the retorts are charged (usually once in' 24 hours) with mixed zinc ore and coal. The part of the charge to be first Worked oil' is the ring of charge immediately adjacent the heated inner Wall of the While this ring of charge is worliing ofi', its temperature will not rise materially above the temperature at which the particular charge actively reduces, due to the .fact that the heat energy is consumed in bringing about en the reduction. "When` however, the ring of charge is largely Worked-0H, the temperature of the worked-otf charge will rise because there is little, if any, other heat absorption taking place. As the temperature of this :J5 ring rises heat begins to iiow more rapidly into the next inner ring of charge Where it in turn is absorbed at the active temperature of reaction level. By thus progressively workiing-oli the inner concentric rings of charge and further heating and raising the temper ature of the outer rings of spent charge o r residue; the working-oit of the charge ultimately progresses to the center or core 'of the retort. l

The progressively workedoi rings of charge consist for the most partof exhausted coal ash and exhausted zinc ore. The exhausted coal ash, which is very loose and cellular as a consequence of the carbon being burned out of it, and theexhausted zinc ore,

VWhich is also very loose and cellular as a consequence of the zinc being volatilized out of application led March the core ot' the retort through this insulating residue and into the core in a given time. iin the case of the ordinary zinc or spelter retort (6 to 9 in diameter) operating with a loose or unagglomerated chaige, this is a days (approximately 24 hours) operation. But .even with this length ot' time allowed tor the penetration of the heat, the temperature of the retort wall necessary to driye the heat into the core of the charge is so high that the exhausted ore and coal ash in the outer ring next to the ret-ort Wall fuse and slag to the Wall of the retort, thus causing bridging and hanging up of the charge, lowering the hea-t conduc-V `tivity of the retort Wall and making the removal of the exhausted charge diiiicult.

The spent residues are removed from the retorts with considerable diiiculty and have to he manually pulled or scraped from the retort with a specially constructed tool. These spent residues are frequently slagged and form very undesirable adhesions to the Walls of the retorts. ln addition to the labor difficulty of charging and discharging the retorts and the long time element-24 hours for the complete Working ott' of a retort charge-this heretofore customary practice and burning of these retorts take relatively lona` periods of time. y.

The progress of recent yearsin the commercial smelting of zinc ores to produce zinc metal has been largely in two directions, viz: the use in furnace construction of refractories more resistant to high temperatures and the Working of less easily fusible charges. The development of the commercial art in these directions has been motived by the general appreciation that the higher the temperature level at which heat can be driven into a charge the more charge can be put into a given sized retort and the more complete the reduction and volatilization of the Zinc content of the ore. i

In order to utilize heat at higher temperature levels, it was first necessary to construct the heating chambers or laboratories, in which the retorts are placed and in which the heat is generated, of more temperature resistant materials. Accordingly better tire clays were used for the retort supports and chamber arches, and then entirely different refractories, namely, silica and silicon carbide, were substituted for fire clays. The best practice to-day is to use silica arches where the brick iscooled on one side and silicon carbide center walls where the entire brick is in the hotzone. At the same time that the heating chamber was being made more temperature-resistant, the retorts were also being improved. The use of better clays and more particularly the increased use of silicon carbide has resulted in the production of retorts which will stand much higher temperatures than formerly Without sagging or slagging awa f.

The second move to permit the use of higher temperatures in zinc smelting has been in the direction of improving the ore and carbonaceous reducing fuel in the mixed charge with the view of making the charge less fusible. With respect to the ore, this has been accomplished by better milling practice in which more of the readily fusible and barren constitutents are separated from the zinc minerals. Better blending of different ores has also brought about a lessened tendency for the mixed charge to fuse at the prevailing Asmelting temperatures. K. The carbonaceous reducing fuel has been improved by selection of materials of relatively low ash content, as Well as by jigging or tabling carbonaceous materials of relatively high ash content.

Consequently, we find that temperature levels of zinc smelting operations have materially increased in the past ten years, particularly in the more progressive smelters. In contrast With this full appreciation of the advantages of higher temperature operations and the commercial realization of these advantages, zinc smelting at lower temperatures and particularly temperatures below .i200o C. has received little, if any, serious commercial consideration either in the field or iii the literature. This is undoubtedly due to the fact that it has not heretofore been considered possible to Work-0E an economical size of charge for the production of zinc metal even in a 24 hour cycle with heat sup-plied at temperature levels less than 1250 to 1400o C. As a matter of fact, these high temperatures are actually necessary if heat is to be driven through a loose and highly insulating charge and residue such as is encountered in the present customary spelter retort practice.

As a resultofexhaustiveinvestigationsand experiments I havelfound that with suitable carbonaceous reducing agents, or suitable reducing and carbon activating agents and u nder proper conditions of iineness of the zinciferous and carbonaceous materialsX and proper mixing conditions, appreciable amounts of zinc are reduced from its compounds at temperatures below 950 C., and that the rate of evolution of zinc becomes substantial at about950o C. and can be brought up to such a point that 90 per cent, or more elimination can be made in less than 2-1 hours at temperatures around 1050o C. rl`he step of making reduction or smelting at such relatively low temperatures entirely commercial, is one of getting heat at that temperature level to a sutliciently large quantity of charge at approxima-tely the same time. This I have found is possible by aggloinerating the charge of mixed zinciferous and carbonaceous materials and utilizing to the maximum extent the three modes of securing heat 4transfer (namely, rapid transfer of heat by heat carrying gases flowing with considerable velocity through the voids of the agglomerated charge, rapid radiationfof heat across the `voids between the agglomerates, and good conduction of heat through individual densilied agglomerates) discussed at lengthin the copending patent application, Serial Number 103,902filed January 27, 1927.

My present invention accordingly involves reducing or smelting an agglomerated charge of mixed zinciferous and carbonaceous reducing materials by supplying the heat necessary for reduction ata temperature level or potential not in excess of 1150o C. Vith such relatively low operating temperatures, I have moreover discovered tha-t thereduction may be conducted in reducing chambers in which such Wall or walls thereof as are exposed to the hot charge and the hot gaseous products arising therefrom are constructed of metal, more particularly wrought iron, nickel-chromium, nickel-iron, cliromiuin-iron and nickcl-iron-chromium alloys. Accordingly, the invention further involves reducing or smelting an agglomerated charge of mixed zinciferous and carbonaceous reducing materials in a metallic reducing chamber wherein the heat necessary for reduction is supplied at a temperature level or potentinal not above 1150C.

I have found that two conditions are of particular significance in making commer- ',ifcrous and carbonaceous materials and so conducting the reducing operation in a chamber of such dimensions' and configuration as to obtain the fullest possible advantage of the rapid transfer of heat throughout the charge by currents of hot gases. The second condition involves making the mixed ycharge of zinciferous and carbonaceous materials as easily reducibleas possible, and thereby lowering the temperature at which zinc is evolved-at a given rate. This may be eii'ected in several ways, as for example, by activating the carbonaceous reducing agent, as for example by caking carbonaceous caking materials, or by the addition of carbon activating agents like water vapor,hydrogen alkaflies. salt, iron oxide and the like, or by adding hydrogen, water vapor, ammonium chloride, zinc chloride and the like to the gases passing through the charge, or by more thorough comminution of the zinciferous and carbonaceous materials, or by more intimate admixture of these materials.

Il'hile the advantages of high temperature. smelting, or reduction, as hereinbefore pointed out-,are important and have accomplished considerable economies in practice. a consideration of the possible economies that can be obtained by low temperature reduction makes the latter practice decidedly attractive for many reasons. In the first place, low temperature reduction, when proper regard is taken of mechanical and thermal stresses, makes possible the use of metal walls for the reducing chamber, with all the advantages that metal has due to its ability to be machined, bolted. welded or otherwise fittedtogether into assemblies of considerable size. V

The second important. advantage of low temperature reduction. and perhaps of even greater importance than the first. I have mentioned, is that it makes possible the working of cheap low grade ores for therecovery of zine metal, which have notheretofore been considered availablefor such purpose, due to their tendency to fuse down. slag, bore through the retort or actually run out of the front of the retorts like water, when smelted at the prevailing relatively7 high temperature levels. In the practice of my present invention. I have found it possible at the relatively low temperature levels which I utilize to eliminate or more of the zinc present in the most refractory and'low grade zinciferous )roductsthat vI have encountered, and this has been accomplished by bringing the agglomerated charge progressively through the reducing chamber without any fusion or s1agging whatever, and discharging the still agglomerated residues in what the zinc meta-llurgist would call a dry condition. An im portant factor closely related to the aforementioned second advantage of low temperature reduction is the fact that alkalies and other materials that have a tendency to lower the fusing or slagging temperature of the .charge also have a marked beneficial effect upon the rate at which the zinc is reduced at low' temperatures. By working at low temperatures, the good effect of the alkali and similar additions may be realized without the harmful after effects of slagging the eiihausted residue. y

A further important advantage of low temperature reduction is that atthe lower temperatures there is considerably less vola tilization of materials which interfere with the subsequent condensation of zinc vapor and lessen the purity of the zinc metal collected. Thus, such volatile substances as sulfur-oxygen compounds, silicon and sulfides of zinc, lead and iron have a tendency to coat or deleteriously1 modify the surface of the condensed droplets of zinc in the condenser, and thereby seriously interfering with the coalescence of these droplet-s.

Another important advantage of low temperature reduction is the fact that the lower the temperature of the heating and reducing chambers the more economical will be the utilization of the heat from whatever source it may be supplied. Furthermore, such relatively low temperatures permit the u se of less temperature refractory and consequently less costly and in many cases better heat-insulating materials of furnace construction.

In the accompanying drawings I have illustratcd two different types of furnaces adapted for the practice of the invention. It is to be understood that the drawings are il lustratory. and thatthe invention may be practiced in other types of furnace. In the drawings Fig. 1 is a front sectional elevation and Fig. 2 is a side sectional elevation of a ver. tical retort furnace,

Fig. 3 is an enlarged sectional elevation of the top of the furnace of Figs. 1 and 2 and the associated condenser,

and

Fig. 5 is a transverse sectional elevation of a horizontal traveling hearth furnace.

The vertical retort furnace illustrated in Figs. 1. 2 and 3 of the drawings comprises a vertically disposed and suspended cylindrical retort 10 of metal, such as wrought iron. The retort 10 is surrounded, for the greater part of its length. by a heating chamberl or laboratory 11. The heating chamber 11 is built within a furnace structure comprising an outer steel shell or casing 12, a. layer of Fig. 4 is a longitudinal sectional elevation powder 13, an intermediate lining 14 of silo-cel brick or the like, and an inner lining 15 of one or more layers of brick or the like. Appropriate openings are provided through the wall of the furnace structure permitting the insertion of pyrometers 16 Within the'heating chamber 11, for ascertaining Vand appropriately' controlling the temperature throughout the length of this chamber.

The furnace structure is mounted on an appropriate foundation 17. A cylindrical extension 18 is bolted, or otherwise appropriately secured, to the underside of the,bottom steel plate of the furnace structure and serves as a guide for an extension of the retort 10 belo'w the bottom of the furnace structure. A pan conveyor 19 is operatively mounted directly beneath the extension 18 and is Ladapted to withdraw the worked-0R charge or spent residue from the bottom of thenetort 10 and convey the same from underneath the furnace structure to appropriate means of discharge.

Any appropriate means may be employed for externally'heating the metal retort 10, due regard being had for the particular metal of which the retort is made. Thus, for example, the products of combustion from burning fuel, such as coal, oil, or gas, may be conducted through the heating chamber 11 around the metal retort 10 and to an appropriate stack. Heat may be generated in the metal retort itself, as in an electric induction furnace, the metal retort acting as the absorber and converter to heat of the electromagnetic energy radiated from a surrounding primary electric circuit. 1n the apparatus illust-rated in the accompanyino drawings having a Wrought iron retort, the heating of the retort is preferably effected by graphite resistors. The electric heating units comprise threepairs of graphite resistors 20 positioned at different levels within the heating chamber 11. The resistors 20 are hollow for an appropriate length thereof and have a spiral slot so as to provide a helical resistance path for the flow of the electric current. The resistors 20 of each pair are connected together on one side of the furnace structure and at the opposite side of the furnace structure the two resistors are connected respectively to opposite terminals of the source of electric energy.

The resistors 2() are supported Within carbofrax tubes 21. The ends of these tubes rest on shelves in the opposite walls of the furnace structure and the centers of theftubes are supported onshelves 22. The tubes 21 have openings 23 in the top thereof.. This arrangement of the resistors within the tubes produces controlled and uniform heating throughout the length of the retort 10.

The top of the metal retort 10 extends some distance above the top of the furnace structure. It is important that this extension of the brackets. -a circular plate 28 resting on the I-beams 25 the retort 10 be carefully insulated to prevent undue radiation of heat therefrom and, more particularly in the case of a wrought iron retort, be protected against oXidizin influences. The extended upper portion o the retort 10 has a lateral outlet pipe 10, also of metal and preferably welded to the retort. A graphite cylindrical sleeve 24 fits snugly within the outlet 10 and serves to conduct the gaseous products generated in the retort 10 to a multi-channelled condenser.

The meta-l retort 10 is hung or suspended in any appropriate manner from I-beams 25 of the structural frame work of the furnace. A series of metal brackets 26 are bolted or otherwise appropriately secured to the top of the retort 10, with asbestos felt or other ap- A propriate heat insulating material 27 disposed between the surface of the retort and r1`he brackets 26 'are secured to and' having a central opening of somewhat greater diameter than the diameter of the retort 10. Asbestos felt or other appropriate heat insulating material 29 is preferably disposed betwcen the adjacent surfaces of the brackets 26 and the plate 28.

- A metal ring 30 is bolted to the retort 10 von the inside thereof and above the outlet 10.

Heat insulating material 31, such as asbestos, is interposed between the ring 30 and the retort. An inverted sheet-metal funnel 32 rests on the ring 30 and the space between the retort and the funnel, above the ring 31, is filled with hardened carbon paste 33.

A truncated conical plug or bell 34 is movably suspended within the flared bottom portion of the inverted funnel 32. The plug 34 is preferably made of graphite with its lower surface recessed and filled with heat refractory material 35. An operating rod 36 is secured at its lower end to the plug 34 and is adapted to be raised and lowered for charging the retort as more fully explained hereinafter.

The upper extending portion of the retort 10 and its lateral outlet 10 are surrounded by hardened carbon paste 38. A cover 37, of sheet metal or the like, is provided for the top of the funnel 32, and the lower edge or rinrof this cover is embedded in dust coal 39 on top of .the hardened carbon paste 33. A pressure responsive device 40 extends through the cover 37 into the upper end of the funneli`32 and is operatively connected to a pressure gauge 41 and a pressure recording instrument 42. The upper extending portion of the retort 10 with its encompassing layer of carbon paste 38 is surrounded by dust coal v43 appropriately confined by a sheet metal `2f ing central orifices vof different (graduated) plication, Serial No. 167,135,5tiled Feb. 10,

1927. This condenser comprises a' gas distributing and molten metal collecting chamber 46 having 'a gas inlet'in communication with the gas outlet 24 fromthe retort 10. A multi-tubular condensing tower 47 is mounted on top of the chamber 46 with its multiplicity of tubes c1' channels in communication with the chamber. -The chamber 46 and tower 47 are built of `graphite and are surrounded by a layer of hardened carbon paste 48,'which in turn is surrounded by .a

mass of dust coal 49 approximately confined by ametal casing 50. A sheet metal hood 51 covers the top or gas exit ends of all the channels in the tower 47. The lower` edgeor v rim of the hood 51 is embedded to a desired l 59 of fire clay or the like. The chamber 46 Q1 extent in the` surrounding dust c'oal 49. The hood has a top opening in which plugs 52havsizes may be inserted. A pressure responsive device 53 is inserted in the hood 51 and is operatively connected to a pressure gauge 54 and a pressure recording instrument 55. The

chamber 46 is provided near its bottom with a tap hole 56 through which molten zinc metal may be withdrawn from the'condenser v from time to time. The tap hole 56 is normally closed by an inner graphite plug 57 secured to a manipulating rod 58 and an outer i plug v is also provided with a` clean-outopening 60 normally closed by an inner graphite plug 61 and outer plug 62. The outer ends` of the tap hole 56 and clean-out opening 60 are preferably covered with heat-insulating caps 63.

The lower endof the metal retort 10 extends downwardly through the bottom lining of the heating chamber 11 and i's not supported by or `otherwise tied into the lining, but on thecontrary is free to move, if it will, with respect tothe lining. This lower end of the retort is sealed about its circumference d and adjacent the bottom lining by a ring of brick, hardened carbon paste, graphite or the like 15 which is non-binding on the retort. It will thus he seen that the retort is supported entirely from its `ton and is at no other place secured to or tied into the furnace structure. The retort is therefore free to extend. expand or contract as it will.

The top of the metal retort is carefully insulated to preventV loss of heat and is further- ,mcrc carefully protected vagainst oxidizing "influences and againstthe alloyingand embrttling action of condensed zinonietal. Thus, while the metal retort is4 red hot throughout that part of its length exposed to the source of heat, and the hot gaseous products of the reduction, its top is relatively cold and black. The upper extending portion of the retort is carefully insulated by asbestos felt or the likeV from all adjacent heat-conducting media, thereby reducing to a minimum the loss of heat byconduction through and away from thisportion of the retort. The hardened carbon paste 38 and 33, surrounding the outer and inner surfaces of the top portion of the retort, acts both as a heat insuLlator (with'respect to metal) and as a maintainer of reducing conditions around the upper portion of theY retort. The charglng structure at the top of the retort effective-- ly protects this relatively cold portion of the retort against the alloying and embrittling inuencesof condensed zinc metal.

' vThe metal retort 10 may be made of vWrought iron, steel, alloy steel and the like.

Hybnlckel Nlchrome Duralloy Chromium 17% Bmg In practicing the invention in the furnace illustrated in Figs l, 2 and 3, the mixed, charge of ,zinclferous and carbonaceous materials is agglomerated and progressively passed through the metal retort or reducing chamber 10.A The agglomeration of the mixed charge, including the size, shape,l

strength and manner of making the ag lomerates, as well as the progression o the agglomerates through the retort are conscribed in the aforementioned application, withthe view of securing the o timum conditions forA the rapid trans er of heat throughout the agglomerated charge by cur?- ucted in accordance with the principles derents of hot gases. Thus, the agglomerated charge passes through the retort, without substantial breaking down 'of the agglomerate A and worked-off residues are discharged from the bottom of the retort as required for the charging thereto of fresh agglomerates; the dischargedv residues being in a dry condition and for themost part inthe form of agglomerates.

In accordance4 with the characteristic feature of the invention, the heating chamber or laboratory 11 of thevfurnaceis maintained at such a temperature that the metal wall of the retort does not exceed about 1150 C. This .is accomplished by close ,ob-servation of the temperatures throughout the heating chamber as indicated by the pyrometers 16 and by lil) careful regulation and control of the heating means. In this manner heat is supplied to the agglomerated charge at temperature levels or potentials not above 11500 C., and, ,in consequence of the favorable conditions existing in the progressively advancing charge for the transfer of heat therethrough by currents of hot gases, the heat supplied at this relatively low temperature level is rapidly` brought to all parts of the charge.

The reduction or Working-off of the agglomerated charge in the retort 10 is a substantially continuous operation. From time to time as required, fresh agglomerates are charged into the top of the retort following the discharge of an appropriate amount of spent residues from the bottom of the retort. In charging fresh agglomerates, the cover 37 is raised and the neck of the charging funnel 32 filled with agglomerates. 'Ihe cover is then lowered to close the top of the funnel 32 and the plug 34 lowered permitting the agglomerates to drop into the retort. The plug 34: is then raised and the sequence of operations repeated until the desired amount of charge has been introduced into the retort. The charging operations are thus conducted with a minimum loss of Zinc carrying gases.

|Ihe primary sealing of the charging end of the retort 'is effected by the plug 34: seating in the flared portion of the funnel 32, and the final seal is effected by the c over 37 with its rim embedded in the dust coal 39 and its top layer of dust coal 45. The metal-laden gases from the retort 10 flow int/o the multichannelled condenser where appropriate temperature and pressure conditions are maintained for good metal condensation in accordance With the principles described in the aforementioned application, Serial No. 163.902.

rEhe following specific example Will further illustrate the principles of the present invention as practiced in an apparatus of the form shown in Figs. 1, 2 and 3, although it is to be understood that this example is merely illustrative and in no sense restrictive of the invention.

The retort l0 was a wrought iron tube 29 feet long` 20 inches internal diameter With a Wall thickness of inch. The outlet pipe 10 was 12 inches in diameter and Was about 3 feet from the top of the retort. rIhe graphite sleeve 24 was Sinches in diameter. The temperature in the heating chamber or .laboratory 11 was approximately 1075 C.

rIhe charge was made up of approximately parts by Weightof finely divided zine silicate ore (containing from l5-50% of zinc) and L 10 parts by Weight of a caking coal (containing about 18% .volatile matter) and 3% of K Waste sulfite liquor f (50%A solids). Approximately of the zinc'ore Y passed through a 20 meshrscreen. IrI`he caking coal (Consolidation Georges Creek Big Vein) was pulverized so that approximately 80% passed through 20 mesh screen. The zinc ore and coal were placed in a revolving mixer of the kind employed for the mixing of concrete and thoroughly mixed. From i the revolving mixer, the material was dumped directly into a dry pan Chilean mill and subjected to the mixing and comminuting action of the mill for some minutes. From the Cilean mill, the material was taken to a briquetting press and formed into briquettes by a compressive force of approximately 2,000 pounds to thevsquare inch. The briquettes were approximately spherical and about 2% inches in diameter'.

The briquettes, Without drying, were charged into an `externally heated vertical coking retort. In this retort, the briquettes were subjected to a coking temperature of about 700o C. In the coking operation, it is desirable-to leave behind in the coked product as high a percentage as possible of the noncondeusible volatile substances, and at the same time eliminate as completely as possible the tars. As a result of the coking action, the intimately mixed particles of ore and coal are firmlyv held together by the binding action of the coke formed in the coking operation. The coke agglomerates Were transferred without substantial loss of heat from the coking retort to the vertical smelting' retort, and introduced therein at the rate of approximately 300 pounds at intervals of 11/2 hours. The heating of the retort was controlled so as to maintain a temperature of approximately 1075o C. on the outside of the retort, and no slag was formed and no fusion of the agglomerates took place.

'l` he gaseous products of the reaction for the most part zinc vapor and carbon monoxide gas With from 0.2-0.8% of carbon dioxide pass through the condenser Where the zinc vapor is condensed. collected and periodically Withdrawn as Zinc metal. Although the charging of the furnacewas intermittent (every 11/2 hours), its operation as a Whole was continuous, the metal retort 10 being at Vall times filled with an agglomerated charge undergoing reduction.

Spent residues were Withdrawn from the bottom of the retort preceding each charging operation in such vamount as required for the subsequent charging of approximately 300 pounds of fresh agglomerates. For the most j part, these residues as discharged retained their original briquetted form and Were free [and non-adherent (what the'zinc metallurgist would call a' 'dry residue), The amount of zinc in these residues averaged about 4%, thus representing an elimination or extraction of over %,of the total zinc in the original ore. 96% yof the zinc eliminated or extracted from the'ore was condensed and collected as slab zinc metal; the remaining 4% '55 proximately one agglomerate deep) of spent l f 1,ssa,sue

y being for the most part recoverable aszinc oxide and blue powder.'

The apparatus illustrated in Figs. 4 and 5 of the drawings comprises a Itravchng hearth 64, ofthe pan conveyor type, constituting the bottom of a relatively long reducing'chamber of rectangular section.

.chamber is embodied in an a'ppropriate/ furnace structure 66 having a heating u'e 67 overlying the top wall or roof- 68 of the reducing chamber. c

. The wall 68 is constructed of metal. This metal wall is preferably built up of a plurality of sections or -units with gas-tight joints. The adjacent edges of the metal sections or units are preferably overlapped and if necessary the joints between sections may be filled with cement in order to make the Wall or roof 68 as a Whole substantially-gas tight. Any of the metals and alloys hereinbefore mentioned as suitablefor the metal Wall of the reducing chamber may be used in the construction of the Wall or roof 68. 'Since this metal Wall bridges between supports, I prefer to construct'it of cast metal, and in order to prevent sagging at the reducing tempera tures, I prefer to provide transverse strengtheningribs 68, at frequent intervals, say' 12 inches apart, throughout the length of the Wall 68.

Fuel burners 69 extend through the side vall (or walls) of the furnace structure into the flue 67, and the hot products of combustion from these burners flow through the flue to an exhaust conduit 7 0 communicating with a stack 71. Pyrometers 7 2 are inserted at appropriate points inthe flue 67 for determinj ying and controlling the temperatureof the heating `gases passing therethrough. j

The pan conveyors, as they enter the reducing chamber travelling in the direction.

indicated by the arrows, are covered with appropriate layer of line material delivered, onto the pans from a. hopper 73 at the charging end ofthe furnace. "Worked-off or spent agglomerates are delivered onto the bed of fines from a hopper 74 adjacent the hopper 73. The spent agglomerates become partially embedded in th'e'ines andto this extent prevent the embedding of agglomerates undergoing reduction. rIlhe agglomerate/d charge of mixed zinciferousand carbonaceous reducingl materials is delivered from a hopper 75, adjacent the hopper 74, onto. the layer (apagglomerates. The layer ofspentvagglomerates interposed between the bedof fines and the overlaying layer of agglomerates undergoing reduction insures the desired porosity of the entire bed of these latter agglomerates, and consequently. the efficient transfer of heat Vby currents of hot gases throughout theI bed of these agglomerates. At theldischar'ge end ofthe furnace additional line material is The reducing invent` ion.

charged from a hopper 76 onto the` spent residues as they emerge from the reducing chamber.

A gas outlet 77 communicates with the top of the reducing chamber 65 near the charging end'and extends upwardly through the vflue 67 and the overlying layer or'layers 78 of heat refractory material. The upper end of the gas outlet 77 communicates through a horizontal passage with the condenser for the' nection with Figs. 1, 2 and 3, and similar elements areindicated by the same reference characters.

The spentresdues and fines are discharged from the pan conveyor onto a grizzly S0 through which the ines pass while the agglomerates slide down the grizzly and are appropriately disposed of. The line material passing through the grizzly is conveyed back to the fine -hoppers 73 and 76 in such relative amounts as required. An appropriateamount of the spent or Worked otl' agglomcrates is conveyed back to the hopper 7 4. The discharge end Vof the furnace -is-enclosed by a sheet metal casing 81 and any dust resulting from the dscharging operation is carried from the casing 81 to the stack 71 or to any other suitable dust collecting system.

In practicing my presentinvention', the apparatus of Figs. 4 and 5 is operated as follows: The temperature in the flue 67 is earefully controlled andl regulated so that heat is supplied to the agglomerated charge in the reducing chamber through the metal wall or *layerof spent agglomerates upon which is superimposed the agglomerated charge to be Worked-off. .The agglomerated charge substantially fills the reduc-ing chamber 65 above the layeryof spent agglonierates and is progressively advanced through the chamber by 'the movement of the pan conveyor. Heat is Atransferred and conducted into the agglomerated charge from thc heated roof or top metal wall 68 by currents of hot gases in the manner characteristic of the aforementioned The mixture ot' metallic zinc vapor and carbon monoxidegas (resulting from the reduction of the zincifcronsmaterial in the a'ggloinrrated charge) passes from the reducing chamber through the gas` outlet T7 into the multi-chzfnnel-ledl carbon condenser.

Where substantially all of the metallic zine vapor is condensed tomolten--zincmetaL From time to time, molten zinc metal is withdrawn from the condenser through the tap hole 5G. A

l'hen the metal wall or walls of the reducing chamber are made of rolled or wrought mctahit (or they) will preferably be hung or suspended from above. Rolled or wrought metal at high temperature has a certain relative lack of stiffness as compared with cast metal at high temperature, but when hung or suspended, the rolled or wrought metal wall may yat high temperatures uninterruptedly expand or stretch downwardly. In this way, sagging, bulging, or other malformation of the chamber wall is avoided` and chamber walls of less thickness may be utilized.

Vhen the chamber wall is made of cast metal, it is generally preferable, due to the weight and stiffness of eastV metal at high temperatures, to support it from the bottom or lower end, leaving the upper end unhampered for expansion and contraition. Using cast metal a thicker chamber wall may be utilized, and this thicker wall combined with the greater stiffness in compression of cast metal permits the use of higher temperatures than is possible when the metal is in tension. lVhen the metal chamber wall spans or bridges between supports, cast metal is preferable.

rlhe metal chamber wall may be made of a single piece of metal or may be made up of' several units or sections appropriately united or joined. `With reducing chambers of con siderable size, it is preferable to use several units or sections in building up the metal wall. lVith rolled or wrought metal these units are preferably welded together, while with cast metal, the units are simply {itt-ed together with lapsed ioints if desired and with an appropriate cement in the joints if necessarv.

The metal walled reducing chamber, in addition to its lower cost and the facility With which large assemblies with side gas outlets and the like may be built. has the further important advantage over chambers built up of heat refractory materials that renewals of the metal wall may be conveniently made without a great loss of time and often without cooling down the furnace. For example, a large non-metallic vertical retort set in its heating chamber takes days to cool down sufficiently so that men can get in and tear it down and build uo another retort, and then days more of careful heating upso asrnot to crack the structure before it is up to the working temperature.V A Worn-out metal retort ori-the other hand, can be readily pulled out of the top of the furnace structure and replaced by a new metal retort without cooling down the furnace, and With only a short ini terruption inthe furnace operation. A

metal retort wl-ich has failed due to a bad section of metal, a local hot spot, a burn-off due to air or any other cause may be withdrawn from the heating chamber and a path or new section welded onto the metal retort and the thus repaired retort replaced in the heating chamber with a minimum shut-down.

Metal has theY further advantage of being able to withstand thermal shocks far better than refractory materials. This is particularly important where intermittent charging and discharging are practiced, and where the freshly charged agglomerates are relatively cold. Metal has another advantage, as contrasted with refractory materials, in that it exhibits little tendency to be slagged or stuck up with adhering residues, and hence offers the minimum resistance to the progression of the agglomerated charge through the reducing chamber.

The manner of heating the reducing chamber determines in some measure the nature of the metal to be used in the construction of the chamber wall. rEhe greatest latitude in the choice of metals is permitted When the heating laboratory for the reducing chamber is lled with highly reducing gases, as. for example, when the laboratory is heated by graphite resistors as shown in Figs. 1, 2 and 3 of the drawings. Under these highly reducing conditions Wrought iron stands up Very satisfactorily. 1When, however, heat is supplied to the'reducing chamber by fire gases (such as from producer or blast fur,- nace gases, oil or coal firing, etc.) it is preferable to construct the metal wall or walls of the reducing chamber. of the nickel-chromium-iron alloys or the like because of their resistance at high temperature toioxidlzing influences. l

By supplying heat to the charge at a temperature level not in excess of 115()o C., 1 mean that this is the temperature of the metal wall of the retort and consequently the maX- imum `temperature of the heat imparted to the charge at any stage in its working-off and including, of course, Worked-od residues to which heat may or must be supplied in the course of the reducing operation. rllhis does not necessarily mean that the temperature in the heating chamber surrounding the reducing chamber may not be higher than 1150o C., since it may be advantageous or even necessary that the temperature of the heating chamber be above 1150o C. in order to drive the heat into the charge at the desired operating temperature level not in excess of 1150 C. For example, it is possible and advantageous in the operation of a vertical metal retort to have the heating chamber at a temperature somewhat above 1150o C., particularly in that part where the charge in the retort contains a relatively high percentage of zinc and where-the'velocity of gas flowing y in the voids of the charge is relatively high rthereto of fresh agglomerates, and with- Lesage@ and consequently the transfer of heat from the retort wall to the center or core ofthe charge is very ra id. Under such conditions the 'metal wall ot the retort due to the rapid removal of heat from itsinside surface may not attain temperatures even of 1150 C., since the heat absorption by the charge within the retort at the temperature level below 1150o C. is as rapid a-s heat can be conveyed to the outside of the retort wall. The faster a charge is passed through such a retort, the

heat dissipation from the upper end of the retort so as to maintain the vapor and gases at a temperature adapted substantially to prevent condensation of zinc on the metal retort whereby alloying of zinc with the metal walls and embrlttlement of the retort is prevented.

In testimony whereof I affix my signature,

FRANK G. BREYER.

higher the temperature in the heatin chaming the agglomerates through an externally heated u right metal retort without substantial breahing down of the agglomerates during their entire passage through the retort; heating the agglomeraties during the course of their passage through the retort to a Sulliciently pounds of zinc and liberate zinc'vapor without slagging or fusing the agglomerated charge, the transfer of heat from the heated wall of the metal retort through the agglomerated charge being eli'ected in large part by currents of hot gases' generated within the charge and flowing through the l voids thereof towards the gas outlet of the retort, discharging spent residues for the most part in the form of agglomerates from said retort as required for the charging drawing fromthe metal retort a gaseous product containing metallic zinc vapor while inhibiting heat dissipation from vthe upper end of the retort so as to maintain the gaseous product at a temperature. adapted substantially to prevent condensation of zinc on the metal retort whereby alloyigh temperature to reduce coniing of zinc with the metal walls and embrittlement of the retort is prevented.

2. In the method of reducing an agglomerat'ed charge of mixed zinciferous material and carbonaceous reducing agent, the-steps which comprise progressively advancingthe.

agglomerates through an upright metal reduction retort as liberated zinc vapor and evolved retort gases rise upwardly through the retort, withdrawing the zinc vapor and accompanying gases from the retort to a zinc lyapor treatment device while inhibiting 

